Switched mode power supplies of the above-described type are generally known, and they are commercially available for several applications.
In one example, the switched mode power supply is implemented as a boost converter, for converting the output voltage of a solar cell array (in the order of 100 V) to a higher constant DC level in the order of about 420 V, i.e. higher than the maximum voltage of the standard mains voltage. With such converter, it is possible to transfer energy from solar cells to the mains.
In another example, the switched mode power supply is implemented as a DC/AC inverter, for generating an AC current from a DC voltage. Such inverter can be used in, for instance, a lamp driver, having an input for connection to AC mains, and having a driver output for driving a discharge lamp. Such drivers typically comprise a stage where a substantially constant voltage is generated from the alternating input voltage, followed by a stage where an alternating current is generated on the basis of said constant voltage.
In yet another example, the switched mode power supply is implemented as a transconductance amplifier for driving an actuator in a motion control apparatus.
Generally speaking, switched mode power supplies have been developed for a specific output power. Generally speaking, for a higher output power, the size of the components used in the power supply must be larger. This can be avoided by using a power supply assembly comprising two or more power supply units connected in parallel. In that case, each individual power supply unit only needs to provide a relatively low power so that the size of the individual components can be relatively small, which implies a reduction of costs. Also, an advantage would be that use could be made of low-power supply units which have already been developed and which have already proven themselves, without the need of developing a complete new high-power converter. Further, it is an advantage that low-power supply units can easily be manufactured, and that high-volume production facilities already exist.
A further advantage of using multiple power supply units connected in parallel is to be recognized in the fact that it is possible to generate an output current with a low ripple amplitude. FIG. 1 illustrates a time graph of a typical power supply output current I, which successively rises (line 101) and falls (line 102) between an upper level IH (line 103) and a lower level IL (line 104). On a sufficiently large time scale, such current can be considered as being a constant current having a magnitude IAV=0.5·(IH+IL), and having a ripple amplitude 0.5·(IH−IL).
In principle, it would be possible to have each power supply unit of a power supply assembly operate completely independently from all the other power supply units. Then, however, it may happen that the units operate in phase, in which case the ripple amplitude of the overall output current of the power supply assembly is the summation of the individual output ripple amplitudes of the individual power supply units. A general aim of the present invention is to have the ripple as small as possible.
Further, a disadvantage of independently operating units is that subharmonics may be caused in the output current, i.e. signal variations having a frequency equal to the difference frequency of the switching of two units. A further aim of the present invention is to prevent such subharmonics as much as possible.
Therefore, it is preferred that the power supply units operate in synchronization, such that their output peaks are distributed evenly in time. FIG. 2 is a graph illustrating this for a case of two power supply units, providing output currents I1 and I2, respectively, in a 180° phase relationship with each other. It can easily be seen that, if the individual currents I1 and I2 have the same amplitude, and if the rate of increase dI/dt from the lower peak to the higher peak is equal to the rate of decrease dI/dt from the higher peak to the lower peak, the resulting current Itotal is substantially constant, having no ripple or only a very small ripple. Even when said individual currents do not have ideal match, typically a reduction of the ripple amplitude is achieved anyway.
Generally, when N represents the number of power supply units, these units are ideally operating in a 360°/N phase relationship with each other.
Operating power supply units in a power supply assembly such that they operate in synchronization but with shifted phases is indicated as “interleaved” operation. Interleaved operation relevant to the field of application considered here has already been proposed in the publication “interleaved converters based on hysteresis current control” by J. S. Batchvarov et al, 2000, I.E.E.E. 31st Annual Power Electronics Specialists Conference, page 655. In this proposal, relating to an assembly of two converter units, one of the converter units has the status of master whereas the other converter unit has the status of slave. The proposed control circuitry of this proposal is rather complicated.